The present invention relates to a field effect transistor, and more particularly to a metal-semiconductor field effect transistor.
A metal-semiconductor field effect transistor (MESFET) is generally designed such that at least a buffer layer and a channel layer are formed in order on a semi-insulating GaAs substrate through a crystal growth method that enables an epitaxial film to be formed, for example, a solid source molecular beam epitaxy (MBE) method, and a source electrode, a gate electrode and a drain electrode are then formed on these layers to control the movement of carriers (electrons) in the channel layer due to an electric field strength applied through the gate electrode, thereby controlling a current flowing into the drain electrode from the source electrode.
In order to improve the characteristics of the MESFET, there has been known that it is effective to restrain the diffusion of carriers into a lower side (substrate layer side) of the channel layer. To achieve this, there have been proposed a p-layer-buried structure having a buried p-layer ("Oki Denki Research & Development", Vol. 64, No. 1, pp. 51-54, January 1997), a structure having an undoped AlGaAs buffer layer made of a compound the general formula: Ga.sub.1-x Al.sub.x As (Japanese Patent Application Laid-Open No. 53-126282 (1978)), and so on.
In order to study the FET characteristics of an MESFET with a structure having the above undoped AlGaAs buffer layer, the present inventors manufactured the MESFET having the above structure for trial, and investigated the drain current to voltage (ID-VD) characteristics. As a result, the present inventors have observed the hysteresis and kink which are presumed to be caused by impurities such as hole traps existing in the undoped AlGaAs buffer layer, and found such a problem that excellent ID-VD characteristics cannot be obtained.